1. Field of the Invention
This invention relates to an expandable coupling, and in particular to an expandable threaded coupling for connecting expandable tubulars.
2. Description of the Related Art
The gas and oil exploration and production industry has recently begun to employ expandable downhole tubulars, for example in the form of diametrically expandable casing, liner and sand screens. A number of expansion methods have been proposed, including the use of rotary expansion tools and devices. The present applicant has developed a range of rotary expansion tools which feature rollers mounted on a body. Such a tool is rotated within the tubular to be expanded, with the rollers in rolling contact with the inner wall of the tubular. The rollers are arranged to reduce the wall thickness of the tubular by cold work, with a corresponding increase in the tubular diameter.
The majority of downhole tubular strings comprise a number of threaded sections which are coupled together at surface to a desired torque. Typically, each section will feature a leading threaded male portion, or pin, and a threaded female portion, or box, on the other end such that a coupling between two adjacent sections features a threaded pin extending into a corresponding threaded box. Such couplings tend to operate satisfactorily on conventional applications, where the coupling is not subject to expansion. However, expansion of tubular strings including such couplings is problematic. One difficulty that may be encountered is that if a string of tubulars is expanded, for example by using a rotary expansion tool or a cone expansion tool or the like, the torque held in the couplings is released. Furthermore, where expansion is achieved using a rotary expansion tool, the tool applies a torque to the tubulars, such that in certain circumstances it is possible for rotary expansion of a coupling to result in backing-off of a threaded coupling, and thus compromise the integrity of the coupling, or even lead to separation of the coupling, and loss of the string below the coupling.
This has been addressed by ensuring that the thread configuration is compatible with the direction of rotation of the rotary expansion tool, such that the tool tends to tighten or torque-up the coupling as the tool passes into the lower tubular section which carries the box. However, the applicant has found that this tightening or torquing-up of couplings can, in certain circumstances, lead to other difficulties.